An African approach for Risk Reduction of Soil Contaminated by Obsolete Pesticides

Large amounts of pesticides have been shipped to Africa for locust control from the fifties of last century, but did not arrive on the proper place or proper moment thereby becoming obsolete. Stockpiles of these pesticides have created a serious problem and The Africa Stockpiles Programme (ASP), launched by FAO, is designed to rid Africa of stockpiles and to dispose them in an environmentally sound manner (ASP, 2009).

In July – August 2007, an investigation mission was organized by FAO pesticide management programme, in collaboration with Wageningen University and Research Centre and the relevant national counterpart institutions of the Ministries of Agriculture and the Ministries of Environment in Mali and Mauritania. In the project, three sites in Mali and three sites in Mauretania have been visited in the summer of 2007 and investigated.

High concentrations of pesticides were found in soils on the stockpiles. From a risk-based point of view, contaminations are only a risk if they are or may become available. Based on the results obtained and results of analysis of the samples taken, risk reduction proposals have been made. All proposals are based on stimulation of the possibilities of biological degradation of the pesticides in combination with isolation and preventing rain water to transport the pesticides both vertical as horizontal. The results have been discussed in May 2008 and the first implementation has been started in Molodo (Mali) in July 2008.

Decontaminating polluted land is a complex and costly challenge even for the most developed of countries. Dealing with these problems in Mali and Mauritania is almost unthinkable. FAO therefore set about trying to develop a low-tech, low cost solution that could be taught and applied in these countries to reduce the most serious of risks from pesticide contamination.

Working with experts from the University of Wageningen, the first step was to develop a methodology for assessing the scope of the contamination at each site and the risk that this contamination posed to health and the environment. Every contaminated site is unique in terms of the nature and level of the contaminants and the environment in which they are found. Several samples from each site were taken and sent to local laboratories for analysis. The same samples were also analysed at accredited labs in Morocco to cross check the precision of the local lab results.

On the basis of the analytical results and environmental surveys, a site specific remediation plan was developed for each location. Remediation was typically based on addition of organic matter and land-farming to enhance local biodegradation of pesticides. In cases where a high environmental risk existed and the contaminated were considered to be too persistent to leave in situ, the contaminated soil was isolated in treatment cells.

The principle of all treatment was to facilitate, and where possible accelerate the natural degradation of the pesticides in the soil. In the case of organophosphate and carbamate insecticides, this has demonstrably been very successful. Organochlorine insecticides are persistent and degrade very slowly. Where these are present in high concentrations and pose a high risk, the soil has been isolated, adsorbent carbon barriers (made from local activated charcoal) have been used and in some low risk cases, the soil has been left in place. All sites are regularly monitored and this is showing very encouraging results already. In Mali, land-farming started in July 2008 at Molodo. Four months later, laboratory analysis of samples from the same locations showed a decrease by two orders of magnitude of organophosphate concentration in the soil. As anticipated, dieldrin concentrations at the same site remained unchanged.

Work on this methodology continues in Mali and Mauritania and will be piloted in other countries when funds permit. We hope that we will be able to provide developing countries with a viable strategy for analysis and risk assessment of pesticide contaminated sites, together with a suite of tools to reduce health and environmental risks from such contamination quickly and cheaply.